Modern mobile radio systems (e.g. Long-Term Evolution (LTE) and LTE-Advanced) are characterized by a frequency reuse factor of one. This means that all base stations (BSs) can use the full system bandwidth to transmit and receive data. A frequency reuse factor of one is considered to be very efficient from a spectrum usage point of view. However, for the so called downlink transmission from the BS to the mobile station (MS), a frequency reuse factor of one means that the MS receives interference from all active neighboring BSs. If an MS is located at a position where it receives high interference power, this can lead to a degradation in the signal to interference and noise ratio (SINR) at the receiver of the MS. The result can be a low performance (especially in terms of data throughput) or service interruptions at the affected MS.
Coordination between BSs can be a solution for such problems. A set of different coordination schemes has been developed. The state of the art includes transmission point blanking, coordinated beamforming, coordinated scheduling and joint transmission (e.g. Lee, D., Seo, H., Clerckx, B., Hardouin, E., Mazzarese, D., Nagata, S., & Sayana, K. (2012). Coordinated multipoint transmission and reception in LTE-advanced: deployment scenarios and operational challenges. IEEE Communications Magazine, 50(2), 148-155. doi: 10.1109/MCOM.2012.6146494).
Transmission point blanking in combination with coordinated scheduling is considered in the following, as they offer advantages for practical implementation, in particular low requirements in terms of latency and bandwidth in the backhaul network, and low overall implementation complexity.
A framework for coordination between BSs is described in US 2012/0027108 A1. BSs of a coordination cluster exchange information about the transmission schemes they intend to use in the future (as shown in FIG. 9 of US 2012/0027108 A1). Furthermore, US 2012/0027108 A1 focuses on decentralized schemes. The BSs of a coordination cluster by default have the same rights. Based on a certain metric one BS then becomes the “leading base station” for a certain part of the frequency band (e.g. paragraph [0104] of US 2012/0027108 A1). A second architecture which enables coordination is the usage of a central entity (Scenario 2 in Table A.1-1 of 3GPP Technical Report Coordinated multi-point operation for LTE physical layer aspects (Release 11), Version 11.2.0 (available at www.3gpp.org)). The central entity is the master of the coordination and provides decisions or recommendations on how a group of BSs should act.
With respect to transmission point blanking in combination with coordinated scheduling, the master of a cooperation, being it a central entity or a leading base station, makes recommendations or decisions on the usage of radio resources (RR, corresponds to a resource block in LTE) in the system as it is described in the following.
Reference is now made to the system according to FIG. 1, which shows an exemplary mobile radio network 10 with two BSs 12, 13. A central coordinating entity 11 (called coordinator in the following) is connected to the BSs 12, 13, e.g. through the backhaul link via an optical fiber. At each time instance, both BSs 12, 13 can make use of two RRs (RR1, RR2) independently. In the example, each BS 12, 13 serves one respective MS 14, 15. MS 14 is served by BS 12 illustrated by a respective serving link and MS 15 by BS 13 illustrated by a respective serving link. Furthermore, a BS 12, 13 interferes the respective MS 14, 15 of the other BS 13, 12. BS 12 interferes MS 15 illustrated by a respective interfering link and BS 13 interferes MS 14 illustrated by a respective interfering link, if both BSs 12, 13 transmit on the same RRs (RR1, RR2). The interference can be avoided, if a BS 12, 13 does not make use of a certain RR. The following examples shall illustrate the possibilities:
1. BS 12 transmits to MS 14 on RR1 and RR2. At the same time BS 13 transmits to MS 15, also using RR1 and RR2. This leads to a situation where both BSs 12, 13 can make use of the full bandwidth but high interference might occur, i.e. both MSs 14, 15 are interfered on both RRs by the other BS 13, 12.                2. BS 12 transmits to MS 14 on RR1 and BS 13 to MS 15 on RR2. Due to the orthogonality of the RRs, no interference occurs. As BS 12 does not transmit on RR2, MS 15 does not receive interference on this RR2. The same applies for BS 13 and RR1 where MS 14 receives no interference. However, the bandwidth that BS 12 and BS 13 can use is limited to half of the system bandwidth. This example for interference avoidance is especially useful for situations in not fully loaded systems, where the BSs 12, 13 do not require the unused RRs.        3. In special cases a reduction of interference is needed to guarantee a certain level of network quality. As an example MS 14 could be located at a position where the interfering link from BS 13 to MS 14 is strong. In consequence the signal to interference and noise ratio (SINR) at MS 14 drops below an acceptable level. The low SINR can lead to a service interruption for MS 14. It can therefore be required that BS 13 does not use the RRs which BS 12 uses to transmit to MS 14 in order to reduce the interference at MS 14.        
The task of the coordinator 11 is to maintain an overview about the situation in the network 10 and to coordinate the transmissions of the BSs 12, 13. With respect to coordinated scheduling and transmission point blanking this means that:                1. In case the network is not fully loaded, the coordinator 11 tries to find an optimum RR assignment for the network consisting of the BSs 12, 13 attached to the coordinator 11. This refers to coordinated scheduling and the example 2 given above.        2. The coordinator 11 can make a decision that certain RRs at a BS 12, 13 should not be used in order to reduce interference for MSs 14, 15 attached to other BSs 13, 12. This refers to transmission point blanking and the example 3.        
There are two operational modes for a coordinator 11:                In case a coordinator 11 is provided with real-time (or close to real-time) information about the status in the network 10, especially an information about the path losses and radio channels of the interfering and serving links, a full control is possible. Here the coordinator 11 makes decisions about the assignment of the RRs for the network of the attached BSs 12, 13. The BSs 12, 13 then have to implement these decisions.        It is also possible that the coordinator 11 makes recommendations. It is then up to the individual BSs 12, 13 to follow them or not. This is especially useful, when the coordinator 11 does not have real-time information. In such cases, it could happen that a BS 12, 13 overwrites certain decisions by the coordinator 11 if it has newer information.        
Thus, the previous described framework for coordinated scheduling and transmission point blanking of the state of the art consists of a central entity (coordinator 11) which can coordinate RRs (RR 1, RR 2) in the attached network of several BSs 12, 13.
US 2015/0063222 A1 provides a method and a system to support coordinated scheduling, i.e. methodology to generate and share information to describe the performance of each user under different interference hypotheses, and a net benefit metric to compare and finally select one interference hypothesis between multiple options. Furthermore, US 2015/0063222 A1 includes decentralized and centralized architectures for determining the coordinated scheduling decision. In both cases, it is assumed that all the possible scheduling decisions are available. In the case that a centralized architecture is considered, there are a maximum of 2^M possible interference hypotheses corresponding to M base stations belonging to the cooperation cluster. Generating all possible interference scenarios and evaluating the one with the maximum benefit as proposed is not efficient and might not be possible in polynomial time. Additionally, decisions made on one PRB might or might not influence the possible decisions on other PRBs. If they influence the decisions on other PRBs, it would imply to generate and compare the scheduling decisions on the basis of all PRBs, i.e. 2^(M*L) possibilities, with L the number of PRBs.